It's essentially a resource management thing. Fully claustral queens have to raise their first workers without any additional supplies - they just have the reserves in their gasters and the wing muscles (they get recycled) to feed their workers - so their first workers are smaller (and thus cheaper) than the regular workers.
However it isn't proven that these "nanitics" have any shorter lives than regular-sized workers. It's more likely that they die more often for other reasons (getting eaten, exhaustion, accidents) because they do not have the strength in numbers the later generations can muster.

Semi-claustral and parasitic queens might not have nanitics at all (or at least not to such an extend), also semi-claustral queens are often only marginally bigger than their workers.

"We should respect all forms of consciousness. The body is just a vessel, a mere hull."